Aldaric acids are a group of sugar acids, where the terminal hydroxyl or aldehyde groups of the sugars have been replaced by terminal carboxylic acids. These acids are characterized by the formula HOOC—(CHOH)n-COOH, with n being an integer of from 1 to 5. These dicarboxylic acids, on account of their combined functionalities, are interesting chemicals. E.g., as sequestering agents, corrosion inhibitors or monomers for making polymers made on the basis of dicarboxylic acids, such as polyesters or polyamides. Preferred aldaric acids are those wherein n is an integer of from 3 to 5. Aldaric acids of particular interest are those derived from C5 and C6 sugars, like xylaric acid, glucaric acid, mannaric acid, gularic acid and idaric acid. An aldaric acid of particular interest is galactaric acid, the aldaric acid corresponding to the sugar galactose. Applications for galactaric acid range from sequestering agents (Kohn et al. Collect. Czech. Chem. Commun. 1986, 1150) to building blocks for polymers (e.g. Moore & Bunting Polym. Sci. Technol., Adv. Polym. Synth., 51). Other aldaric acids of particular interest are glucaric acid, mannaric acid and gularic acid, aldaric acids that can be obtained from the carbohydrate fraction of biomass sources including pectins and a variety of different seaweeds.
In WO 2013/151428 a method is disclosed to produce aldaric acids by the oxidation of the corresponding uronic acid, wherein a starting material comprising the uronic acid is subjected to oxygen under the influence of a supported gold catalyst and in the presence of a base. An interesting aspect of this method, is that it makes it possible to unlock the chemical potential present in the form of uronic acids in hemicellulosic streams, which are typically obtainable from biorefineries.
Biorefineries serve to conduct the sustainable processing of biomass into a spectrum of marketable biobased products and bioenergy. A biorefinery is an installation that can process biomass into multiple products using an array of processing technologies. In general, biomass coming from plants, will result in streams based on lignin, cellulose, and hemicellulose, respectively. Hemicelluloses can be removed from biomass, e.g. by treatment with hot pressurized water. This results in formation of water soluble oligomeric and monomeric sugars and their dehydration products such as furfural and hydroxymethyl furfural. Another source of hemicelluloses is in the agro-food industry. Whilst hemicelluloses, in theory, are a source of a wide variety of useful chemicals, it is desired to find methods to make better use of this potential, by providing economically attractive processes to harvest such chemicals therefrom. A particular interesting hemicellulosic feedstock from the agro-food industry comprises sugar beet pulp, a by-product of the sugar beet industry. Sugar beet pulp contains a high content of pectic substances, being composed of arabinose and galacturonic acid as the main monomers. Other sources of pectins are all kinds of different fruits, including e.g. apples, carrots, cherries and citrus fruits, especially citrus peels. Another potential source of uronic acids is being formed by alginates. A large variety of seaweeds including red and brown seaweeds like Laminaria digitata, Saccharina latissima and Ulva lactuca contain huge amounts of alginates being composed of mannuronic acid and guluronic acid as the composing monomers; after hydrolysis of the alginates such monomers can be used as feedstock for the production of aldaric acids.
Whilst the process disclosed in the aforementioned reference is highly suitable, it would be desired to be able to convert the uronic acids under acidic, rather than basic circumstances. This would bring about a substantial advantage in that the stream of raw materials by nature is neutral to acidic, so that the addition of large amounts of base can be dispensed with. Also, the desired end-products, viz. aldaric acids, can only be isolated as a free acid by ultimately adjusting the pH to acidic. Thus, a process in which the whole oxidative conversion would take place under acidic conditions, would avoid both the use of additional base and the generation of a relatively large amount of salts as a by-product of ultimately acidifying the reaction product, so as to isolate the free aldaric acid.